Found out I might have unknowingly cheated after term completion

Assuming that the no-collaboration rule was really not communicated to you, then you didn't cheat. You followed the rules as you reasonably understood them based on the information you were given. There's no such thing as "unknowingly cheating"; it requires intent.

However, there is a substantial risk that the professor may think you have cheated, if he mistakenly thinks he made it clear to the entire class that collaboration was not allowed. So it is in your best interest to clear it up before any formal charges are considered. Just explain what you've said here: you were told that the rules would be "the same" as the midterm (if you have this in writing, better yet), and you later found out that other students were told something different.

The fact that the term has ended means nothing. It is entirely possible, in all academic systems I know of, to impose a punishment for cheating even after the term is over, perhaps months or years later. And it is not too unusual that investigations can't be completed before grades are due, in which case the professor may assign the grade based on an assumption of no cheating, but it can be changed retroactively if his investigation confirms wrongdoing.

So even if you don't want to "dig it up", the professor still can, and you will convey more honesty if you get out ahead of it. You have a good defense, but it won't look as good if you have to say "well, I found out afterwards there might be a problem, but I didn't say anything because I hoped you wouldn't notice".

Note that your acquaintance definitely did cheat, if he's telling the truth about what the professor told him: he'd been clearly informed that collaboration was not allowed and he did it anyway. Moreover, he did it in a way that placed innocent students at risk of being suspected and prosecuted for cheating. Many people would feel that you also have an obligation to report this to the professor.

This is an uncomfortable situation.

Document your knowledge of the rules as you understood at the time of the final, and also the midterm (the rules to which were included by reference on the final). Save any emails or written explanations you received about the rules.

I would (somewhat painfully) recommend that you not bring this up with your instructor at this time. If they are as disorganized and reckless as this situation suggests, then I would not trust their capacity to process or deal with the mess in a reasonable way. By analogy: There are cases of people reporting software/system bugs in good faith who are then brought up on charges by confused authority figures looking for someone to punish.

Note also that you only have a report of this issue second-hand from a single student who is self-admittedly an unreliable source. There have been many times in my experience when a student says,"Professor X said [crazy thing]", and on tracking it down, turns out to be a misunderstanding or misrecollection on the student's part. Alternatively, people who tend to be fraudsters also tend to lie and cause anxiety for personal amusement. So you may be liable to bother or confuse your professor about nothing, and be embarrassed by that.

If the issue does come up in the future, then present your documented evidence honestly.

Don't poke the bear.

I agree with the answer by Daniel Collins, which quotes the famous saying "Don't poke the bear". Your description of the situation makes it very clear that you did not intend to cheat, and that you are proactively coming here to ask advice on how to handle the situation, which many would say is quite responsible of you to do, so there is only two reasons why you would want to poke the bear:

• If you want to experience what it is like to be involved in a university-level academic integrity investigation (trust me, you do not).
• The bear is more likely than not, to come after you anyway, so you wish to proactively "turn yourself in", which would improve your chances of having a happy ending to the situation.

Let's asses the second point in more detail:

For the final, the professor simply said that the rules would be the same as the midterm listed above. To our discredit, no one double checked what the professor meant and simply went with it.

In my opinion, with only the information you have provided here, you were under no obligation to "double check". If the professor said the rules would be the same as for the midterm, leave it at that.

If anything, all our answers were extremely similar (most were exact copies) to the point that it would be obvious we were collaborating.

This part would be seen as cheating in my opinion. Even if the professor said that "you can collaborate", it is common sense that, on a take-home exam, "being allowed to collaborate" still means you have to write your own separate answers.

If you did submit extremely similar answers to anyone else, then I personally would call it cheating, and you might consider telling your professor in advance, to mitigate any consequences you might have coming at you for it, except:

• You said it has been 4 weeks since it happened and nothing has been said of the situation, so I would guess that nothing will happen.
• You said that the class thought the rules were the same as the midterm: Do you know if your group submitted almost identical answers during the midterm too? If you did and there was no consequences, it's unlikely (though not impossible) to be brought up now.

If you did not submit extremely similar answers to anyone else, then my personal advice to you is to relax and start to focus on more important things, such as your career goals or next set of academic courses.

In the future: please do not submit what you described as "extremely similar answers" for anything, even if it is a take-home exam. There is no excuse not to spend a bit of extra energy to make each answer your own, and if you cannot do this then you do not understand your answer, and should not be trying to give the examiner the false impression that you do. This will keep you from having to deal with the stress of your current dilemma, ever again.